The goal of the proposed competing continuation study is to implement and evaluate an enhancement and modification of the SHIELD HIV prevention intervention targeting sexually active injection drug users (DA13142) to examine its efficacy, diffusion to main risk network members, and sustainability. Baltimore has some of the highest rates of injection drug use (IDU), HIV, and sexually transmitted infections (STIs) in the U.S. Interventions that are culturally competent and target drug users' main sex and drug partners may be especially effective for HIV/STI and HCV control and prevention. Grounded in a social influence framework, with theories of social diffusion, social norms, and social identity, the SHIELD intervention uses peers to promote risk reduction within their social networks. Results indicate greater reported sex and drug risk reduction and HIV advocacy with network members at 6-month follow-up among experimental compared to equal-attention control participants. The findings also suggest that the intervention had limited effect on participants' risk behaviors with their main sex partners. Based on extensive outcome and process evaluation of the SHIELD study and reviews of the literature, major changes proposed are: adding two training sessions with index participants' main risk ties to increase their receptivity to indexes' outreach messages, and incorporating booster sessions to enhance the sustainability of intervention effects. The proposed study design is a controlled phase II efficacy trial. Index participants randomly assigned to the experimental condition will be trained to conduct peer HIVIHCV education and distribute prevention materials among their main high-risk network members. Index participants (n=400; 200 per arm) will be street-recruited sexually active injection drug users who are able to recruit at least one risk network member. Indexes will be asked to recruit risk network members (average 2; 400 per arm) for intervention and assessment. One risk partner per index will be selected for recruitment for 2 dyadic intervention sessions, with priority to main sex partners; the other network members will receive only the pre- and post-assessments to measure diffusion of intervention effects. Outcome evaluation will include pre-test and 3-, 6-, 12-, and 18-month post-intervention assessment. Main outcome measures will be self-reported sex and injection risk behaviors. Self-reports will be cross-validated by urinalysis of STIs. [unreadable] [unreadable]